The bug appears to be that this dialog needs to make the built-in templates read-only (worked in v19 but not since). There's no support for modifying Installed Templates. For now, just don't modify templates in the "Installed Templates" folder. You can create new templates in the "User Templates" folder.
I was hoping there was an easy way to create a new user template from an existing template but it doesn't look like you can do that.